1. Field of the Invention
The present invention relates to economical inertial navigation units (IMU's) for short range, relatively low accuracy guidance applications such as munitions. More particularly, this invention pertains to a multisensor of the type that employs paired triads of accelerometers mounted upon counteroscillating platforms for directly measuring linear accelerations and for determining rotation rates from Coriolis forces with respect to a three-axis system.
2. Description of the Prior Art
IMU's measure space-dependent rotations and accelerations with respect to orthogonal space axes. Their design is beset by numerous difficulties as this requires the simultaneous measurement of six independent variables. For example, gyroscopes of the ring laser and fiber optic type require a lasing cavity dedicated to each input axis, mandating a total of three lasing cavities, an expensive undertaking, for obtaining three of the six measurements required of an IMU. (An example of a laser device for measuring rotation about three axes is shown in United States patent, property of the assignee herein, Ser. No. 4,795,258 of Martin entitled "Nonplanar Three-Axis Ring Laser Gyro With Shared Mirror Faces".) IMU's employing spinning wheel gyros must deal with such gyros' limitation to measurement of rotation with respect to two axes of measurement, necessitating the use of an additional drive mechanism for the third input axis. Again, this does not in any way account for the additional complication introduced by the remaining measurements of accelerations with respect to the axes.
Simplicity and economy are particularly significant in the design of IMU's for munitions guidance and like applications. Such uses are characterized by non-reusable payloads, limited flight durations and only moderate accuracy requirements. One economical type of system for measuring both rotation rates and linear accelerations with reference to a set of three orthogonal axes is a multisensor mechanism taught in a series of United States patents, also the property of the assignee herein (Ser. Nos. 4,996,877 entitled "Three Axis Inertial Measurement Unit With Counterbalanced Mechanical Oscillator"; 5,007,289 entitled "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator"; and 5,065,627 entitled "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator.") The devices disclosed in the referenced patents employ piezoelectric drive mechanisms for causing a pair of counterbalanced platforms to oscillate out-of-phase about a common axis within a housing or case. Accelerometers, housed in a vacuum to avoid the effects of gas damping, mounted to radially-directed elements of the platforms provide measures of both linear acceleration and rotation. The latter (rotation) values are derived from the (Coriolis) forces sensed by the accelerometers at the resonant frequency of the counteroscillating structure.
The accelerometers of the above-described multisensor arrangements consist of substantially-planar, compliantly-hinged paddle-like masses. For example, the multisensor of U.S. Pat. No. 4,841,773 teaches hinges of paired crossed-beam flexure blades. The accelerometer hinges are compliant structures designed for sensitivity over a wide band of frequencies to permit complete nulling without excessive torquer voltages. The nulling voltage is a measure of not only the low frequency (in the range of 0 to approximately 500 Hz) linear accelerations but also the much higher frequency (about 3 kHz) Coriolis accelerations that indicate rotation rate. Separate capture loops are required to generate and measure the torquing voltages for nulling the responses of the pendulous mass to both low frequency linear accelerations and to the Coriolis acceleration at the counteroscillation frequency.
Arrangements of the above-described type are subject to a number of shortcomings. Due to the necessarily-high compliance (and, therefore, low stiffness) of the hinges for broadband closed-loop operation, the vacuum-housed accelerometers, and, hence, the multisensor, are subject to saturation through shock and vibration forces. Such saturation occurs when the pendulous mass is angularly displaced by a transient shock to such an extent that the separation between it and one of a pair of torquer electrodes becomes sufficiently small that the resultant electrostatic force can draw the undamped mass through the remaining gap. Once stuck to the electrode, the mass cannot readily revert to a stable nullable configuration in this totally-undamped, highly destabilizing electric field, rendering the multisensor non-operational. Time-consuming and difficult resetting and recalibration procedures are then required to free the pendulous mass and bring the multisensor back on-line.